FM

Frequency Modulation:


A fundamental feature of a modular synth is the ability to modulate one module (or one parameter on a module) with another. By patching a VCO signal to the FM input (i.e. the pitch CV input) of another VCO the first VCO is shaping the second one every time the oscillator cycles – it is modulating it at audio frequencies.

by patching the audio frequency output of one VCO (the ‘Modulator’) to the CV input governing the frequency of another VCO (the ‘Carrier’), changes to the amplitude in the Carrier are so fast that the waveform passes from a plain audio frequency signal with varying pitch (such as results from patching an envelope or applying an LFO) to an entire new audio waveform that is significantly different to the original waveform.

The new waveform sounds so different to the original is because Frequency Modulation generates additional component frequencies, not always harmonically related to the frequency of the Carrier or Modulator, in the output signal (the results of the peaks and troughs of the Modulator amplitude shaping the peaks and troughs of the Carrier waveform).

Why are these extra frequencies so relevant?

With FM the 'shape' of the resulting waveform (in other words, the sound), is affected not just by the frequency of the Modulator, but also the by the amplitude of the modulating waveform (there are still amplitude variations in the wave modulating the Carrier - with no amplitude component to a wave there would be no wave!). The sound therefore is dictated by both frequency and amplitude of the Modulator, resulting in a massive variation in the sound from small changes in the Modulator.

This means that just patching the output of a VCO to the FM input of a second VCO generally produces metallic, ‘clangy’ sounds – often discordant and similar to ring- modulation. This can be great, but sometimes you may want sounds to be more musical. To be able to play an FM sound up and down the keyboard in conventional manner it will require both the Carrier and the Modulator to track the keyboard equally so that any extra frequencies (and their associated amplitudes) produced in the output waveform remain harmonically consistent.

This may seem like a difficult thing to achieve since as the Modulator frequency increases, these extra frequencies and amplitudes change. For example, if you play one octave higher, although modulation frequency doubles, the extra frequencies and amplitudes etc. might be halved or changed even more unevenly. To make the amount of modulation consistently track the keyboard the amplitude needs to be affected by the FM CV source – i.e. we need to introduce a VCA in the signal path (VCAs allow modulation of amplitude!). Therefore we must patch the FM Modulator source to the VCA CV input that dictates the amplitude of the Modulator. I believe this cunning trick was dreamed up by a fella named John M. Chowning.



The drawbacks (advantages?) of this configuration is that as well as needing envelopes in the signal path to shape the output VCA, they are needed between the Modulator frequency CV source and the modulation VCA to create any real complexity to a timbre – this requires mixing the Modulator frequency CV source with an EG using a CV mixer before applying the output of this to the VCA. But, this also allows for incredibly powerful control over a sound generated from only one pair of VCO/EG/VCA setups.

These blocks of Modulator VCO/EG/VCA feeding a Carrier VCO/EG/VCA are called operators. Just a single pair of operator and Carrier output setups can give massively complex sounds (and cv’able ones at that!). Now consider stacking multiple operators in a chain – each FMing the one after! Unfortunately, even with a large modular synth, only a few operators can usually be implemented before the setup becomes far too demanding as far as VCA/VCO/EG numbers will reasonably allow – that is, more than a couple of operators is often unrealistic. Other fun options remain though – inserting ring-modulators, using VCO sync, and filters etc

There is a further issue with such FM synthesis patches though; if you patch modules in an FM configuration as shown at the top of this page, not only does the modulation index shift hence introducing 'unwanted' sidebands that create relatively 'non-musical' klangourous tones, you will also find that whenever pitch changes at the 1V/Oct input of the carrier VCO there will be a degree of 'glide' - the pitch will slew from one note to the next, with more severe artefacts of this nature occuring as the change in pitch increases.



Fortunately, another synth genius, Bernard A. Hutchins Jr. has given us a patch to counteract this effect fairly successfully by using an inverse envelope cv multiplied by itself using either a VCA or ring modulator to pull the frequency of the carrier back down when it would normally detune:



To use these two techniques together (Chowning's pitch tracking modulation index and Hutchings' anti-glide) the patching get's somewhat complex and you'll have to do a fair bit of attenuating/adjusting envelope levels etc., but it is possible. Navs also reports that this patch exposes a DC offset inherent in many VCOs that may require the output of the VCO be run through an AC processor such as a filter.

Although this feels slightly like betraying the principles of modular synthesis and the rewarding thrill of patching a solution to a tricky issue, nearly all of this complex patching can be simplified, avoided or even improved upon through the wonders of the Linear FM inputs present on some VCOs or even thru-zero FM synthesis (though this is still an expensive route if you wish to achieve it with analogue oscillators...)